Leap Year 2012: Why We Need February 29

Leap day added to correct "sloppy" calendar drift.

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Musician Graham Nash (left) and his wife celebrate her Leap Year birthday in February 2000.

Photograph by Joe Raedle, Getty Images

PUBLISHED February 29, 2012

Wednesday, February 29, marks leap day—an extra 24 hours that gets added to the month of February roughly every four years—that is, every leap year—to keep the modern calendar in line with celestial cycles.

Early calendars were often based on lunar months, which average 29.5 days. But a year of such months totals only about 354 days.

This discrepancy resulted in annual events—festivals, agricultural milestones, religious observations—drifting out of alignment with their intended seasons as the years passed.

"Civilizations like Rome would add months to try to correct the drift of the lunar calendar," said David Ewing Duncan, author of the book Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year.

But Duncan describes the Roman solution as "sloppy."

"It played havoc with everything from religious holidays to market times," he said.

"Remember, this is a sophisticated society. You had rents due, interest accruing on loans, all kinds of things that would be moved out of shape."

"Finally it became so ridiculous that Pope Gregory XIII was convinced by his astronomers that basically all the Christian holidays were being celebrated on the wrong days," Duncan said.

The pope introduced his Gregorian calendar in 1582, which determined that only one out of every four "century years" would observe a leap year. Thus while the years 2000 and 2400 are leap years, 2100, 2200, and 2300 are not.

The Gregorian calendar was gradually, and sometimes grudgingly, adopted by much of the world and remains in common use.

China's Leap Year Solution

In China the Gregorian calendar is commonly used, but the traditional lunar-solar calendar is still observed to determine the dates of festivals such as Chinese New Year.

As with other ancient calendars, the Chinese traditional calendar corresponds to the phases of the moon.

But the Chinese system also includes a solar calendar and introduces an entire leap month about every third year to keep the calendar in synch with the seasons.

Chinese leap years of 13 months have 383, 384, or 385 days.

Such a system preserves a monthly cycle that begins with the new moon and centers on lunar events, which are important for the timing of religious and cultural milestones.

Maya's Missing Leap Year

The ancient Maya, famed for their elaborate and accurate calendar systems, observed two calendar years, but neither seemed to have bothered with a leap year.

"As far as we know, the people of Mesoamerica—the Maya included—didn't care about leap years," said Anthony Aveni, an expert in ancient Mesoamerican astronomy at Colgate University.